1. Field of the Invention
The present invention relates to a zoom lens having a magnification function which changes a field angle by changing a focal length, in particular, to a zoom lens suitable for a digital camera, a video camera and the like, which obtain digital image data of a subject by using an imaging element, to an imaging device using such a zoom lens as an imaging optical system, and to an information device such as a personal digital assistant having such an imaging function.
2. Description of the Related Art
There has been a significant growth in the digital camera market, and users have a wide variety of demands for digital cameras. Particularly, users always desire high quality picture technology and downsizing technology, which are the main demands of digital camera users. For this reason, the high quality picture technology and the downsizing technology are also required for a zoom lens which is used as a shooting lens.
Regarding the downsizing technology, at first, it is necessary to reduce the entire length of lenses in use (distance from a most object side lens surface to an imaging plane), and it is also important to reduce the entire length in a collapsed state by reducing a thickness of each lens group. Regarding the high performance technology, resolution corresponding to an imaging element having at least 10 million-15 million pixels is required for the entire zooming range.
Moreover, a wider field angle of a shooting lens is required by many users, and a half-field angle of a zoom lens at a short focal end is preferably 38 degrees or more. The half-field angle of 38 degrees corresponds to a focal length of 28 mm in a case of a silver-salt camera using a silver salt film (i.e., Leica film) having a width of 35 mm. Furthermore, a high magnification ratio is also required. The zoom lens having a focal length corresponding to about 28 to 200 mm in a 35 mm silver salt camera conversion (about 7.1 times) enables all general shooting.
As a zoom lens for a digital camera, various types of zoom lenses are used. As a zoom lens suitable for a high magnification, there is a zoom lens including, in order from an object side, a first lens group having a positive refractive power, a second lens group having a negative refractive power, a third lens group having a positive refractive power and a fourth lens group having a positive refractive power, in which when changing a magnification from a short focal end to a long focal end, an interval between the first lens group and the second lens group is increased, an interval between the second lens group and the third lens group is decreased, and an interval between the third lens group and the fourth lens group is changed.
The zoom lens of this type includes, for example, a zoom lens in which a first lens group is fastened when changing a magnification and a zoom lens in which a first lens group reciprocates in arc having a convex shape on an image side when changing a magnification. In this type, if a large displacement of the second lens group which largely shares a magnification function is ensured, an aperture stop disposed near the third lens group separates from the first lens group even when the zoom lens is at the short focal end. Accordingly, the size of the first lens group is increased for a wide angle and high magnification zoom lens. Therefore, in order to achieve a wide angle, high magnification and small zoom lens, it is preferable for the first lens group to move so as to be located closer to the object side at the long focal end than the short focal end. By reducing the entire length of lenses at the short focal end compared to the long focal end, the size of the first lens group is prevented from increasing and a significant wide angle can be achieved.
Since the second lens group is constituted as a variator having a main magnification function, the constitution of the second lens group is very important. As a known second lens group, a second lens group including, in order from the object side, three lenses of a negative lens having a large curvature surface on the image side, a negative lens having a concave surface on the object side and a positive lens having a convex surface on the object side, which are arranged in order from the object side, is described in, for example, Japanese Patent Application Nos. 2008-145501, 2006-23531 and Japanese Patent No. 3328001.
Moreover, as a known second lens group, a second lens group including three lenses of a negative lens having a large curvature surface on the image side, a negative lens having a convex surface on the object side and a positive lens having a convex surface on the object side, which are arranged in order from the object side, is described in Japanese Patent Application No. 2009-198798.
However, the zoom lens described in each of Japanese Patent Application Nos. 2008-145501 and 2006-23531 is not a high magnification zoom lens of 8 times or more. Moreover, the second negative lens from the object side and the positive lens in the second lens group are not joined, so that the eccentric amount of these lenses is increased, resulting in a decrease in the resolution. On the other hand, the zoom lens described in Japanese Patent No. 3328001 has a high magnification. However, the entire length of lenses at the long focal end is increased, so that a small zoom lens is not achieved.
Furthermore, the zoom lens described in Japanese Patent Application No. 2009-198798 is not a high magnification zoom lens of 8 times or more. In this zoom lens, the second negative lens from the object side and the third lens in the second lens group are not joined, so that the eccentric amount of the these lenses is increased, resulting in a decrease in the resolution.